I am familiar with the SP-2550 but I know nothing about the SPX-2550. If it is the same as the SL-70 I know a little about the version made by Maxon but sold by GE in the US as the "Monogram".

The radio does cover 136-174 but only with realignment to operate in about a 7 MHz bandwidth or up to 12 MHz with some degradation. The US versions came factory tuned to the center of either 148-160 or 160-174 and it was necessary to retune if you were not near the center of the band.

Did your radio work before you changed it to the 2M band and what frequency was it on before? I suspect it will need the VCO retuned for any operation and a full TX/RX alignment for good performance unless it was already tuned for the 144-148 range.